Baking displacement map: Base mesh is included in the highres output
Answer from @Secrop on blenderartist.org:


  Add a subdivision modifier to the low poly object before baking.
  
  The thing here is that each poly in the lowres object is flat, but the surface from > the highres is curved… This makes the displacement value at the center of those polys to be greater than their corners.
  If your lowres is later to be subdivided with ‘Simple’ method, this bake will be correct. On the other hand, if you use ‘Catmull-Clark’, then you need to use this subdivision at the baking phase also.


Result: